Java Multithreading and Concurrency Fundamentals

Java provides multiple ways to create threads: extending Thread, implementing Runnable, or using Callable with Future for tasks that return results. However, the best practice is to use thread pools via ExecutorService, which reuses threads and controls concurrency. ThreadPoolExecutor is configured with core pool size, maximum pool size, keep-alive time, a work queue (LinkedBlockingQueue, SynchronousQueue, etc.), and a rejection policy (AbortPolicy, CallerRunsPolicy, etc.). Thread safety is achieved through several mechanisms. The synchronized keyword provides mutual exclusion using monitor locks — it can be applied to methods or code blocks. volatile ensures visibility across threads by forcing reads/writes to main memory, but does not provide atomicity for compound operations like i++. For more flexibility, ReentrantLock offers features like tryLock with timeout, fair locking, and multiple Condition objects. The Java Memory Model defines happens-before relationships that guarantee visibility: unlock happens-before subsequent lock, volatile write happens-before volatile read, and thread start happens-before any action in the started thread. The java.util.concurrent package provides higher-level utilities: AtomicInteger for lock-free atomic operations using CAS, CountDownLatch for waiting on multiple threads, CyclicBarrier for synchronizing threads at a common point, and ConcurrentHashMap for thread-safe map operations. Understanding these tools and choosing the right one for each scenario is what separates junior developers from experienced concurrent programmers.